Optimistic parents often produce more offspring than can survive, with "core" offspring kept but "marginal" ones usually discarded. Motivations for this include unpredictable resources, insurance against developmental problems, and incidental benefits from larger broods. Blackbirds, marsupials, rabbits, plants, and humans illustrate some of the varied ways parents may favour offspring: among marsupial antechinuses, for example, males only live one year but daughters are potential competitors, so younger mothers favour sons and older ones daughters.
Genetic conflicts have significant medical consequences for human pregnancy. Imprinted genes, whose expression differs between paternally and maternally inherited copies, play a role in preeclampsia and gestational diabetes. Human chorionic gonadotropin (HCG) is produced by embryos to try to prevent miscarriages that would be beneficial for the mother; in response mothers have reduced sensitivity and the result has been an evolutionary arms race, a side effect of which may be pregnancy sickness. Down syndrome is more common than other chromosomal abnormalities because Down's embryos produce HCG in huge quantities.
Forbes digresses to look at brood parasitism in birds, in cuckoos and cowbirds.
Human mothers screen eggs during ovulation and embryos during implantation and gestation: most miscarriages occur early and are an adaptive response to chromosomal defects. Older mothers appear to relax this screening, which together with declining embryo quality results in a higher incidence of birth defects; an apparent increase in spontaneous abortions may result from their delay and clinical detection.
Surveying the history of twinning rates, the paternal heritability of dizygotic twinning, and its links with taller and fatter mothers and with Down syndrome, Forbes argues that "twins are maladaptive but a strategy of twinning is not — its adaptive value lies in providing a hedge against defective embryos. Twin births are a cost, contretemps to be avoided." He supports the "vanishing twin" hypothesis, that most human twins are reduced early in development, at such high rates that humans should be considered "obligate brood reducers".
Brood reduction in birds is often carried out by siblings, sometimes facultatively but often obligately:
"a baby black eagle if it is the second hatched in the nest — the beta chick — faces a life that is poor, nasty, brutish and decidedly short. Its sibling, four to seven days older, greets it with repeated blows to the face and body. Each chirp or movement triggers another beating, whether the parents are present or not. At one nest, closely observed, the alpha chick delivered nearly three hundred pecks over beta's first 24 hours, another six hundred during the second day, and more than six hundred on beta's third and last day of life."
Clones and haplodiploidy are genetic spurs to cooperation within families, while individuals that know they are dying or will not reproduce can instead work for relatives. Forbes briefly looks at social insects, but also at sibling synergies in birds and mammals, which are often driven by thermal considerations: he describes an experiment on herring gulls which directly measured embryonic and parental preferences for egg temperature.
Eating offspring can make sense, and in some species surplus offspring or eggs are created deliberately for use as food. Infanticide by step-parents is common. Female lions may try to save their offspring when new males take over the pride, but female rats can't get around the ability of male rats to time gestation so they spontaneously abort to avoid wasting resources. And the massively higher incidence of child abuse by human step-parents is well documented.
The "brave new world" of assisted reproductive technologies offers us gender selection, increasing numbers of dangerous twin births, and the possibility of cloning. Here Forbes gets quite worked up about the dangers of the extra multiple births accompanying assisted reproduction: "Any medical procedure for which a multiple of six fetuses is a possible outcome is clearly corrupt."
Forbes introduces chapters and some sections with "parallels" from literature and history: Darwin's life, Charlotte Brönte, Septimus Severus' sons, Jacob and Joseph, and so forth. And he occasionally ventures into sociobiology, getting into cultural and historical waters where he is much less at home. For example, he writes:
"Parental favoritism has played a key role over the course of human history. Primogeniture in Viking society forced younger sons to seek their fortunes overseas, and between the eighth and eleventh centuries Vikings expanded their trade routes in the south, east, and west."Here we have the all too common move from a banal generalisation to a specific causal claim which is massively simplistic and in this case also just plain wrong, since the Vikings did not in fact practice primogeniture. I actually agree with the core of Forbes' defence of sociobiology and the application of evolutionary thinking to humans, but here and in other places he does his cause poor service.
A Natural History of Families has some repetition and includes some elementary biology. Forbes goes into more detail in presenting specific results, however, and I found much of the medical material new. A Natural History of Families is recommended to anyone interested in evolutionary medicine, wanting a better understanding of pregnancy, or after a genetic perspective on family conflicts.